In book 3 Extermination Alpaharius is stated as using the pale spear which is described as:

"The Pale Spear was one of a number of strange and esoteric weapons associated with the Primarch Alpharius. Rumoured to be a strange Xeno artefact, whose forging predated even the rise of the Eldar, the double-bladed spear seemingly flickered out of phase with the material universe and emitted an eerie and otherworldly howl, when wielded. It was capable of piercing and ripping apart any physical defense it encountered; while against living matter, The Pale Spear inflicted hideously bloodless gaping wounds on its targets, as the flesh where it struck dissolved into oily smoke"

It struck me that this sounds very like a C'Tan phase weapon

"C'tan phase weapons (also known as fractal edged weapons) (including the Callidus Assassin Phase Sword, Cypher's Phase Knife, Necron Warscythes and the metal bodies of the C'tan themselves) are all based around a metal blade of unknown composition that, through the use of highly advanced Necron physics, is capable of slicing through any object irrespective of its physical properties. Energy shields, armour and even daemonic bodies are no defence against a Phase Weapon."